The studies reported in this dissertation are part of the national cohort study pre-COOL to evaluate the developmental and educational effects of early childhood education and care (ECEC) provisions in the Netherlands. More specifically, we evaluated the quality of a large representative sample of ECEC provisions and examined the effects of attending ECEC on the development of two-to-three year old children. ECEC quality can be defined by structural and process characteristics that are thought to nurture child development. Process quality refers to children’s daily experiences and encompasses the social, emotional, physical, and instructional aspects of children’s activities and interactions with teacher, peers, and, materials that are seen as proximal determinants of child development. Structural characteristics, such as group size or teachers’ qualifications, are the distal, more regulable aspects and are considered as important preconditions of process quality. In the current study an internationally widely used observational measure, the Classroom Assessment Scoring System (CLASS) developed in the United States, was used to assess Dutch ECEC quality. In the first study we evaluated the psychometric quality of the CLASS by combining approaches based on classical test theory (i.e., evaluating the factor structure) and item response theory (i.e. evaluating item difficulty and item discrimination). Overall, the psychometric quality of the CLASS was found to be satisfactory, although some problems emerged, possibly pointing to cultural differences in ECEC practices. The second study addressed the relationships between process quality and the structural characteristics, group size, children-to-teacher ratio, teachers’ qualifications, the use of an education program [VVE programma] and, continuous professional development in the center. The findings showed that the emotional quality in Dutch ECEC is moderate to high, whereas the educational quality is low to moderate. Our findings revealed a small effect of ...